The art of measuring the flux of a fluid by an analog method is summarized in U.S. Pat. No. 4,074,572 by Bullis et al, assigned to the assignee of this application, in which a current-measuring technique produces a continuous measurement of fluid flow. Although that method works very well, it is susceptible to DC amplifier drift and calibration problems that inherently affect analog devices. The use of pulsed devices to measure fluid velocity, as opposed to flux, has involved the injection of a pulse of ions that flow along with the fluid, so that the measured transit time is directly related to the fluid velocity. For example, U.S. Pat. No. 2,827,786 and No. 2,637,208 both disclose devices for measurement of the velocity of a gas based upon this principle. If a measurement of the mass flow were desired, using such equipment, it would be necessary to additionally measure the temperature and pressure of the flowing gas.
In the prior art pulsed ion flowmeters, an ion pulse drifted with the gas flow, without the use of an applied drift field. In these prior art pulsed ion devices fluid flow velocity was measured, not fluid flux, which is directly related to fluid mass flow. Further, in prior art radial type ionization flowmeters in which fluid flux is measured, account has not been taken of the impact real world environmental factors have on meter performance; mainly measurement accuracy. The key factor impacting measurement accuracy in ion drift type flowmeters is the question of the identity of the ions that are formed, the mobility of these ions, the stability of the ions and the danger of contaminants giving rise to false readings. These significant problems have been addressed and solved by the invention disclosed herein.